Apparatus and methodology for determining oxygen in biological systems

ABSTRACT

The invention provides apparatus and methods for measuring oxygen tensions in biological systems utilizing physiologically acceptable paramagnetic material, such as India ink or carbon black, and electron paramagnetic resonance (EPR) oximetry. India ink is introduced to the biological system and exposed to a magnetic field and an electromagnetic field in the 1-2 GHz range. The EPR spectra is then measured at the biological system to determine oxygen concentration. The EPR spectra is determined by an EPR spectrometer that adjusts the resonator to a single resonator frequency to compensate for movements of the biological system, such as a human or animal. The biological system can also include other in vivo tissues, cells, and cell cultures to directly measure pO 2  non-destructively.

GOVERNMENT SUPPORT

This invention was made in part with government support under contractnumbers NIH/GM34250 and RR01811 awarded by the National Institute ofHealth. The government has certain limited rights in the invention.

FIELD OF THE INVENTION

This invention relates generally to apparatus and methods fordetermining oxygen tension in biological systems. More particularly, theinvention concerns apparatus and methods for determining oxygen tensionin biological in vivo tissue utilizing physiologically acceptableparamagnetic materials and electron paramagnetic resonance oximetry.

BACKGROUND OF THE INVENTION

Benefits derived from the measurement of oxygen concentrations in tissueare known. Oxygen is the primary biological oxidant, and the measurementof oxygen tension pO₂ (positive pressure of oxygen) can improve theevaluation and understanding of many physiological, pathological, andtherapeutic processes.

Prior art systems and methods for measuring oxygen concentrations intissue are also known, including: the Clark electrode, fluorescencequenching, O₂ binding to myoglobin and hemoglobin, chemiluminescence,phosphoresence quenching, and spin label oximetry. However, thesesystems and methods have certain, and often acute, limitations,especially when used in vivo. They especially lack the qualitiesrequired for complete experimental and clinical use, such assensitivity, accuracy, repeatability, and adequate spatial resolution.See J. Chapman, Radiother. Oncol. 20, 13 (1991) and J. M. Vanderkooi etal., "Oxygen in Mammalian Tissue: Methods of Measurement and Affinitiesof Various Reactions", Am. J. Physiol. 260, C1131 (1991).

The polarographic microelectrode is one popular device for measuringoxygen tension in tissue. However, it has obvious technical difficultiesassociated with the repeated insertion of the microelectrode into thetissue. For example, the microelectrode often damages the tissue, andthere is repeated difficulty in re-positioning the microelectrode at thesame test location. The microelectrode is also relatively insensitive tooxygen concentrations below 10 mm Hg, which is within the requiredsensitivity region for effective oximetry. Finally, the microelectrodemay itself consume oxygen, thereby altering its own environment,inducing measurement errors, and reducing the accuracy and usefulness ofthe evaluation process.

There are scattered reports which concern in vivo pO₂ measurements withsuch devices, especially in skeletal muscle. Whalen and Nair, Am. J.Physiol. 218, 973 (1970), measured pO₂ of cat gracilis at rest using arecessed Au 1-51 μm microelectrode, giving average pO₂ values of 6.6±0.4mm Hg (n=372). Gayeski et al., Am. J. Physiol. 254, H1179 (1988),measured pO₂ of dog gracilis at rest, exhibiting a partial pressurerange of 4.5-35 mm Hg (16.8 mm Hg median), and 95% VO₂ max, using a Mbsaturation technique, exhibiting a partial pressure range of 0.2-2.3 mmHg (0.9-1.8 range of mean). Nevertheless, there are effectivelimitations to these pO₂ measurement techniques. In the microelectrodemethod, for example, it is technically difficult to monitor or make longterm evaluations of pO₂. In the Mb saturation method, it is especiallydifficult to measure low pO₂, and the method can only be used in muscle.

Nuclear Magnetic Resonance (NMR) techniques have been explored andconsidered in the context of oxiometric measurements, especially throughthe use of an oxygen dependent proton hyperfine line in myoglobin andoxygen dependent relaxation of fluorine nuclei. NMR is a commonspectroscopic technique in which the molecular nuclei is aligned in amagnetic field and simultaneously excited by absorption ofradiofrequency energy. The molecular relaxation from the excited stateto the initial state is an observable event that is affected by thepresence of oxygen through exchange or dipolar actions. However, the NMRtechniques have not demonstrated sufficient sensitivity and/orapplicability to the measure of pO₂ in either experimental or clinicalsettings.

Electron Paramagnetic Resonance (EPR) oximetry is another technique formeasuring oxygen tension. Similar to NMR, EPR oximetry is aspectroscopic technique based upon the Zeeman effect and theline-broadening effect of molecular oxygen on the EPR spectra ofparamagnetic materials. These materials have unpaired electron spinsthat are aligned in a magnetic field and excited by microwave energy.The separation between the lower, unexcited energy state and the higher,excited energy state is proportional to the strength of the magneticfield. The presence of oxygen with the excited molecule measurablyaffects the molecular relaxation so that the line width of the EPRspectra changes and provides an indication of pO₂.

Nitroxides exemplify one family of compounds having paramagnetic qualitythat are suitable for EPR oximetry, and which have been used in avariety of in vitro experiments. Although nitroxides have also beentested in vivo, at least two resulting problematic areas exist in suchmeasurements: first, nitroxides tend to be bioreduced; and secondly,nitroxides are relatively insensitive to low oxygen tension levels thatare of the most biological interest today, i.e., less than 10 mmHg.

Other recent discoveries of new paramagnetic materials, such as Fusiniteand lithium phthalocyanine (LiPc), have made progress as oxygen probesin the field of in vivo EPR oximetry. These two compounds, for example,are suitable for in vivo usage because they exhibit certain favorablecharacteristics, including: accuracy; spatial resolution; sensitivity inthe physiologically important oxygen tension range; ease of use; littleor no apparent toxicity; and relative stability in tissues, permittingprolonged measurements over periods of weeks or months afteradministering the compound. Nevertheless, because these paramagneticcompounds have not been previously tested in humans, they will have toundergo very long and extensive toxicological evaluation before they canbe used clinically. This evaluation is likely to be prolonged because ofother problems inherent in the compounds, such as stability andinertness, which encourage indefinite, unwanted persistence within thetissue.

There are other existing problems limiting the effectiveness of EPRoximetry, including the inability to measure EPR spectra efficiently andeffectively, especially in vivo. Conventional EPR spectrometers, forexample, typically utilize microwave frequencies, e.g., 9 GHz, that arestrongly absorbed by tissue and water, and which reduce the useful depthpenetration and measurement sensitivities within the tissue. Prior EPRspectrometers also cannot effectively measure EPR spectra from abiological system such as a live animal, because movements of the animalchange the observed EPR spectra. This movement increases noise andreduces the accuracy. Finally, conventional EPR spectrometers have theresonator and the sample under test, e.g., tissue, within a commonmagnetic field. This constrains the EPR measurement/flexibility, beingsubject to physical size considerations, and potentially to thepatient's dexterity.

It is accordingly an object of this invention to provide an improved EPRspectrometer and associated methodology that are free of theafore-mentioned difficulties.

It is another object of this invention to provide an improved apparatusand method that enables the direct measurement of oxygen concentrationin biological systems, such as tissue.

It is a further object of the invention to provide improved methodologyand apparatus for in vivo EPR oximetry.

Other objects of the invention will be apparent from the followingdescription.

SUMMARY OF THE INVENTION

The invention attains these and other objects, according to one aspect,by providing a method for evaluating oxygen tensions in a biologicalsystem, including the steps of (1) introducing physiologicallyacceptable paramagnetic material to the biological system, (2) applyinga magnetic field and an electromagnetic field to the biological system,and (3) determining the EPR spectra of the biological system. Theparamagnetic material is of the type which has an EPR spectra responsiveto the presence of oxygen, such as India ink, constituents of India inkhaving paramagnetic quality, carbon black, and other carbon-basedmaterial. The biological system includes in vivo and in vitro biologicalsystems, biological tissues, cells, cell cultures, animals, and livehuman beings.

In another aspect, the method provides for the step of calibrating theEPR spectra of the paramagnetic material by comparing the EPR spectra ofthe biological system with the EPR spectra of the paramagnetic materialin the presence of a known concentration of oxygen. Preferably, both themeasured spectra from the biological system and the calibration spectraare determined by the spectra's peak-to-peak line width. Thepeak-to-peak line width indicates oxygen tension in the biologicalsystem, and oxygen concentration is determined directly by comparing themeasured line width to the calibration line width.

In other aspects, the method provides for sweeping the magnitude of themagnetic field between approximately 100 and 500 Gauss to acquire theEPR spectra through the frequencies of the EPR resonance. The step ofsweeping preferably occurs in less than 60 seconds.

In another aspect, the magnetic field includes a first magnetic fieldhaving lines of force in substantially one direction, and the methodprovides for applying a second magnetic field to the biological systemthat is substantially parallel to the first magnetic field. The secondmagnetic field is thereafter slowly varied to modify, or sweep, themagnitude of the first magnetic field between approximately 1 and 500Gauss, to acquire the EPR spectra through the EPR resonance frequencies.Alternatively, an electromagnet is employed to sweep the magneticintensities. Preferably, a third magnetic field is applied to thebiological system that is substantially perpendicular to the firstmagnetic field. The third magnetic field is modulated betweenapproximately 1 and 500 kHz, to improve the signal-to-noise ratio fordetermining the spectra. Preferably, the electromagnetic field appliedto the biological system is directed substantially perpendicular to thefirst magnetic field with an oscillating frequency between approximately100 MHz and 5 GHz, such as in the microwave L-band.

In still another aspect, the method includes the step of determining theEPR spectra by utilizing an EPR spectrometer that has a resonator and anassociated Q factor. The Q factor is determined and monitored forchange, such that, in another aspect, the Q factor is compensated tomaintain resonant frequency during movements by the biological system,e.g., the tissue or animal.

The method in accordance with the invention also provides forintroducing to the biological system a paramagnetic material that hassubstantially uniform particles with diameters between approximately 0.1and 100 microns. Alternatively the paramagnetic material can include atleast one relatively large particle with a diameter betweenapproximately 100 microns and one centimeter. This relatively largeparamagnetic particle functions as a point source to spatially determinethe EPR spectra in the biological system.

In other aspects according to the invention, the paramagnetic materialis introduced to the biological system by several appropriate methods.In tissue, for example, the material can be injecting directly into thebiological system. If the biological system has a circulatory bloodstream, the paramagnetic material can be introduced directly into theblood stream. Accordingly, the method can include the further steps of(1) changing the blood flow to the biological system or tissue, and (2)determining the change in the EPR spectra to provide a real-timeevaluation of the change in oxygen concentration in the tissue.Additionally, the blood flow to the tissue can be reduced to reduce theoxygen concentration in the tissue.

The paramagnetic material can also be introduced to the biologicalsystem via lymphatics. To derive additional spatial information, theparamagnetic material can also be selectively introduced to a localizedregion within the biological system, thereby indicating oxygen tensionat the localized region. Alternatively, the paramagnetic material isintroduced to a biological system having phagocytic activity, such thatthe paramagnetic material is introduced to the biological system byphagocytosis.

The invention also provides for a method to determine EPR spectra of abiological system having a surface. When the biological system has asurface, e.g., the skin of an animal, the EPR spectra is preferablydetermined from the surface. In other aspects, an EPR resonatorconstructed in accordance with the invention for use with an EPRspectrometer directly measures EPR spectra from the surface.

In another aspect, a method is provided for evaluating oxygen tension ina cell. Physiologically acceptable paramagnetic material--which has anEPR spectra responsive to the presence of oxygen--is first introduced tothe cell, such as through phagocytosis. A magnetic field and anelectromagnetic field are then applied to the cell, and the peak-to-peakline width of the EPR spectra of the cell is determined. Theparamagnetic material can include carbon black, carbon-based material,India ink, or ingredients of India ink having physiologically acceptableparamagnetic quality. The electromagnetic field preferably has afrequency between approximately 100 MHz and 5 GHz.

The method additionally provides for the steps of determining the EPRspectra peak-to-peak line width of the paramagnetic material in thepresence of a known concentration of oxygen. The spectra from the knownconcentration of oxygen is then compared to the spectra of the cell todetermine the oxygen tension present in the cell.

The invention also provides a system for determining oxygenconcentrations in biological systems, including (1) physiologicallyacceptable paramagnetic material in the biological system, and (2) anEPR spectrometer to determine the EPR spectra of the biological system.The paramagnetic material can include India ink, an ingredient of Indiaink having physiologically acceptable paramagnetic quality, carbon-basedmaterial, and carbon black. The biological system can be in vitro and invivo biological tissue, biological tissue having phagocytic activity,one or more phagocytic cells, living animals and humans. Theparamagnetic material is introduced to the biological system via anappropriate manner, including: direct injection into the biologicalsystem; direct injection into the blood stream; via lymphatics; andthrough ingestion.

Preferably, in another aspect, the system includes means for determiningthe peak-to-peak line width of the EPR spectra. This line width is thencompared with the peak-to-peak line width of the EPR spectra of theparamagnetic material in the presence of a known concentration ofoxygen. A system according to the invention also preferably includes amagnet, for applying a magnetic field to the biological system, andmeans for sweeping the magnitude of the magnetic field betweenapproximately 100 and 500 Gauss. The magnitude is typically varied in aperiod less than sixty seconds.

In another aspect, a system according to the invention includes means,e.g., a magnet or an electromagnet, for applying a first magnetic fieldto the biological system that has lines of force in substantially onedirection. The system further has means, e.g., a magnet or anelectromagnet, for generating a second magnetic field with lines offorce substantially parallel to the first magnetic field to modify andsweep the magnitude of the first magnetic field between approximately 1and 500 Gauss. Preferably, the system has means for generating a thirdmagnetic field, with lines of force substantially perpendicular to thefirst magnetic field, wherein the third magnetic field is modulatedbetween approximately 1 and 500 kHz to improve the signal-to-noise ratioof the measured spectra.

In still another aspect, the system has an oscillating electromagneticsource for applying electromagnetic radiation to the biological system.The electromagnetic radiation, preferably within the range 100 MHz to 5GHz, such as the L-band microwave frequencies, is directed to thebiological system and is substantially perpendicular to the magneticfield.

In still another aspect according to the invention, the EPR spectrometerhas a resonator and means for determining the resonator Q. Preferably,the resonator Q is compensated in response to movements of thebiological system to maintain the resonant frequency.

In other aspects, the paramagnetic material of the system issubstantially uniform, with particle diameters between approximately 0.1micron and 100 microns. The paramagnetic material can also be one ormore relatively large particles with diameters between approximately 100microns and one centimeter. These relatively large particles functionmuch like a point source for the spectra in the biological system. Inone aspect, for example, the paramagnetic material is localized withinthe biological system, thereby providing a selectable spatial indicationof the oxygen tension in the biological system.

In other aspects, the system provides means to determine the EPR spectradirectly from the surface of the biological system, e.g., the skin of ahuman. If the biological system is biological tissue having acirculatory blood flow, the system can include means for changing theblood flow to the tissue and means for determining the change in the EPRspectra, thereby providing a real-time evaluation of the change inoxygen concentration in the tissue. Accordingly, the system can alsoinclude means, e.g., a tourniquet, for reducing the blood flow to thetissue to reduce the oxygen concentration at the tissue.

The invention also provides, in another aspect, a spectrometer for thein vivo measurement of oxygen concentration in tissue. The spectrometerincludes (1) magnets for selectively applying a magnetic field ofselectable strength to the tissue, (2) electromagnetic oscillator forselectively applying electromagnetic radiation having a frequencybetween approximately 100 MHz and 5 GHz to the tissue, (3) detector fordetecting the electron paramagnetic spectra of the tissue, (4) resonatorarranged to maintain a substantially constant resonant frequency, (5)console in communication with the detector for displaying the EPRspectra, and (6) computer connected to the console for controlling thespectrometer, and for analyzing the EPR spectra.

Preferably, the resonator includes an automatic frequency controlcircuit to tune the resonator to the frequency of the oscillator. Thedetector is preferably arranged with a preamplifier for combined,high-dynamic range detection of EPR spectra.

In other aspects, the spectrometer includes an electromagnetic bridgewith automatic frequency control, a fixed frequency oscillator, and avaractor diode tuned resonator. The electromagnetic bridge, especiallyin the microwave region, is arranged to tune the resonator to theresonant frequency, thereby compensating for movements of the tissue. Inanother aspect, the resonator has a high Q LC circuit coupled with anexternal planar loop via a λ/2 symmetrical line. Further, the computercan be arranged for (1) determining the peak-to-peak line width of theEPR spectra, (2) storing calibration EPR spectra of paramagneticmaterial in the presence of known concentrations of oxygen, and (3)comparing calibration spectra with EPR spectra of the tissue.

In a preferred aspect, the spectrometer system comprises India ink, aconstituent of India ink having physiologically acceptable paramagneticquality, or other physiologically acceptable paramagnetic materials, inthe tissue to be measured.

The methods of the invention preferably utilize an EPR spectrometerconstructed in accordance with the invention, such that the EPR spectrais determined without significant interference from the configuration ormovement of the biological system; and further such that the measurementis compatible with EPR spectra from physiologically acceptableparamagnetic materials, e.g., India ink, in in vivo tissue.

These and other aspects and advantages of the invention are evident inthe description which follows and in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically shows calibration EPR line width spectra of India inkand Fusinite over a wide range of oxygen tensions;

FIG. 1A graphically shows EPR line width spectra from India ink in thepresence of other materials, such as water, serum and oleic acid;

FIG. 1B graphically shows the EPR spectra of India ink in nitrogen andair using a X-band EPR spectrometer;

FIG. 2 graphically shows microwave power and saturation data on lineheight in nitrogen and in air;

FIG. 3 is an EPR spectrometer constructed in accordance with theinvention;

FIG. 4 is a microwave resonator for use in the EPR spectrometer of FIG.3;

FIG. 5 shows the signal response of EPR India ink spectra before andafter restricting the blood flow to the gastrocnemius muscles of anadult mouse injected with India ink;

FIG. 6 graphically shows the de-oxygenation in in vivo mouse muscleinjected with India ink, subsequent to the tightening of a tourniquet;

FIG. 7 graphically shows the de-oxygenation characteristics of mousemuscle injected with India ink over a period of thirty-nine days;

FIG. 8 shows a histological slide of mouse leg muscle forty days afterimplantation by India ink;

FIG. 9 illustrates the tattoo of a human volunteer; and

FIG. 10 graphically shows EPR spectra from a human tattoo based on Indiaink with and without blood flow restriction.

DETAILED DESCRIPTION OF THE INVENTION

The invention concerns apparatus, systems, and methods for determiningthe partial pressure of oxygen, pO₂, in biological systems, including invivo or ex vivo tissue. The invention provides improvements to EPRoximetry by improving the sensitivity, accuracy, and repeatability ofEPR techniques. The invention further provides an EPR spectrometer and aparamagnetic material that are physiologically compatible with in vivomeasurements. This paramagnetic material is already approved for usewith humans; and the material exhibits a measurable correlation betweenEPR spectra and oxygen tension over a clinically effective pressure,sensitivity, and resolution range. These methods, systems, and apparatushave immediate and important application to clinical and experimentalproblems which exist today.

The invention utilizes physiologically acceptable paramagneticmaterials, and in particular carbon black, especially in the form ofIndia ink, as new paramagnetic probes for EPR oximetry. India ink is aninjectable compound that is widely used in clinical settings, with noapparent toxicity. India ink has extensive prior use in humans as thebasis for black tattoos, used for medical purposes as well as forpersonal decoration. It has also been widely used in surgery to tracepathways in tissues. India ink additionally exhibits the desiredphysical and chemical properties required for effective clinical EPRoximetry, having EPR spectra that is very sensitive to the presence ofoxygen. In accordance with the invention, physiologically acceptableparamagnetic materials--such as India ink, constituents of India ink,carbon black, or carbon-based material--are used to directly determinethe pO₂ in biological systems, such as tissue. Previously, no knownparamagnetic material has exhibited the requisite properties to enabledirect, in vivo evaluation of humans.

The description below discusses the relevant properties of India ink,and the methodology and apparatus for determining pO₂ in vivo via EPRoximetry. Experimental results are given from tests conducted with liveanimals, and from tests demonstrating that oxygen dependent changes inIndia ink EPR spectra can be detected in humans. The latter experimentalresults are based upon the presence of India ink within an ornamentalhuman tattoo, and the response of India ink EPR spectra to differingoxygen concentrations present at the tattoo.

India ink is a stable paramagnetic material. It has a single EPR signalspectra with a peak-to-peak line width that is calibrated to directlydetermine oxygen tension pO₂ in vivo. FIG. 1 illustrates one set ofcalibration data in a graph of the EPR spectra line width of India ink20 and Fusinite 22 against pO₂. With reference to FIG. 1, the India inkline width 20 is approximately 600 mGauss in the absence of oxygen andapproximately 4500 mGauss in the presence of air. When India ink iswithin biological tissues, the shape of the EPR spectra is between thesevalues, which is correlated to determine the in vivo oxygen tension. Onthe other hand, over the same partial pressures, the Fusinite line widthonly changed from 500 mGauss at 0 mm Hg to 1200 mGauss at 35 mm Hg.

At least two other noteworthy characteristics are apparent withreference to FIG. 1: first, the India Ink line width spectra issensitive to oxygen tension levels below 1 mm Hg; and secondly, theslope of the India Ink calibration data 20 shows that the EPR spectraline width is particularly sensitive to changes in oxygen tensions ofless than 30 mm Hg, which is a critical realm for effective oxiometricmeasurements. As compared to fusinite 22, for example, theline-broadening effects of the India ink EPR spectra per unit pO₂ aregreater, improving sensitivity.

India ink is additionally less sensitive to the external conditions, andto the compounds present in the biological system under investigation,which might otherwise affect or reduce measurement accuracy. Over thebroad range of conditions that can occur in vivo, for example, theresponse of India Ink EPR spectra to pO₂ is essentially independent ofpH, oxidants, reductants, and the nature or lipophilicity of thebiological medium. FIG. 1A graphically shows the line width of India inkEPR spectra 24 in the presence of various media, including oleic acid25, serum 26, and water 27. The data 24 is the same as the calibrationdata 20 of FIG. 1, to within the accuracy of the measurement.

The experimental India ink data illustrated in FIGS. 1, 1A and 1B, andin the principal experimental data presented in FIGS. 5-8, derive fromIndia ink purchased at SHIKAYA, JAPAN, having a concentration of 80mg/ml. The India ink particles were homogenous in size, and wereapproximately 1 μm in diameter. Other chemicals for the principalexperiments discussed herein were purchased from Sigma, in St. Louis,Mo.

The calibration of India ink and other in vitro experimental studies ofIndia ink were performed on a Varian E-109 EPR spectrometer, which hasan X-band, 9.6 GHz microwave oscillator. Typical control settings forthe Varian spectrometer were: (1) 3210 Gauss of magnetic field strength;(2) 10 mW of microwave power; and (3) a modulation amplitude less thanone third of the line width. Experimental temperatures were controlledwith a Varian gas flow variable temperature control unit. And EPRspectra were collected using EW software, from Scientific Software Inc.,in Normal, Ill., which was installed on an IBM-compatible personalcomputer. DPPH was used as a secondary standard for spin densitymeasurements.

More particularly, the calibration of India ink was as follows. Tenmicro-liters of India ink in PBS was drawn into a gas permeable teflontube from Zeus Industrial Products, Inc., in Raritan, N.J. This teflontube had a 0.623 mm inner diameter and a 0.138 wall thickness, and wasfolded twice and inserted into a quartz EPR tube open at both ends. Thesample was then equilibrated with different O₂ : N₂ gas mixtures. pO₂ inthe perfusing gas was monitored and measured by a modified Clarkelectrode oxygen analyzer from Sensor Medics Co., Model OM-11, inAnaheim, Calif., which was calibrated with pure air and nitrogen. FIG.1B shows that the response of the India ink EPR line width spectra 30 inair, as compared to the spectra 32 in nitrogen, is severe, indicatingthe ink's usefulness for oximetry.

The quantitative dependence of the EPR spectra on pO₂ was obtained bymeasuring the line width as a function of pO₂ in the perfusing gas. EPRline widths are usually reported as the difference in magnetic fieldbetween the maximum and minimum of the first derivative recording of thesignal. In other words, the EPR line width is the peak-to-peakseparation of the first derivative, with respect to frequency, of theLorentzian-shaped absorption spectra.

The experiments presented herein also considered the microwavesaturation effects of the environment. FIG. 2 summarizes microwave powerdata on the line height within nitrogen 34 and air 35. Because powersaturation occurred only at high microwave powers, the in vitroexperimental testing utilized 10 mW of unsaturated X-band microwaveradiation.

With further reference to FIGS. 1 and 1A, the g-value, spin density, andline width of the EPR India ink spectra were measured at roomtemperature. The g-value (2.0027±0.0008) and spin density (2.5×10¹⁹spin/g) of India ink were not affected by oxygen. While the g-value ofIndia ink was approximately equal to Fusinite, the number of spins forIndia ink spectra was more than twice the number of spins for Fusinite(1.0×10¹⁹ spin/g). As illustrated in FIG. 1, the India ink EPR probe isvery sensitive, as compared to Fusinite, at low pO₂, especially lessthan 30 mm Hg of oxygen tension. Conveniently, the principal pO₂dependencies for clinical and biomedical applications occur in the rangeof 0-30 mm Hg pO₂, making India ink EPR oximetry a valuable measurementtool.

India ink EPR spectra exhibited no self-broadening due to changes in theconcentration of India ink particles. No effect, for example, wasobserved in the EPR spectra of India ink in the presence of aparamagnetic agent, K₃ Fe(CN)₆, an oxidant, H₂ O₂, or a reductant,ascorbic acid. The line width of India ink was also not affected byvariation in temperatures between 25° C. and 50° C., nor by variationsin the pH between 4 to 14. FIG. 1A illustrates that the response of EPRIndia ink spectra in the presence of oxygen is essentially independentof the media, including oleic acid 25, serum 26, and water 27.

For in vivo EPR measurements, discussed below, an EPR spectrometerconstructed in accordance with the further features of the invention wasutilized, having a L-band, low-frequency microwave oscillator(approximately 1.2 GHz) with an extended planar loop antennae connectedto a resonator.

FIGS. 3 and 4 illustrate an EPR spectrometer apparatus 40 constructed inaccordance with the invention, and which has significant structuraldifferences as compared to conventional EPR spectrometers. Mostsignificantly, the spectrometer 40 permits the accurate measurement ofEPR spectra from in vivo biological systems, such as live animals, byretuning its resonator 42 to maintain resonant frequency duringmovements of the animal.

A spectrometer 40 constructed according to the invention solves certaintechnology problems which make existing EPR spectrometers incompatiblewith oxiometric measurements using physiologically acceptableparamagnetic materials. Existing EPR spectrometers are especiallyincompatible with in vivo measurements of live beings using paramagneticprobes either implanted in tissue or administered through another route,such as orally, intravenously, or by injection.

The spectrometer system 40 is a low frequency EPR spectrometer thatmeasures the EPR spectra of India ink or other physiologicallyacceptable materials in animals, including humans, and other biologicalsystems. The spectrometer 40 has a resonator 42 and an associatedmicrowave bridge 44. The spectrometer 40 further has a magnet 46,powered by a power supply 48, and modulation coils 50. The power supply48, the coils 50, and the microwave bridge 44 connect to a standardspectrometer console 52. A computer 54 connects to the console tocontrol elements in the spectrometer 40.

In a conventional microwave bridge for an EPR spectrometer, an AutomaticFrequency Control (AFC) circuit locks the microwave oscillator to theresonant frequency of the resonator. This is problematic for the purposeof measuring animals, or a patient, with EPR oximetry. Movements in thesubject being studied cause a retuning of the oscillating bridgefrequency by ±5 MHz, which is equivalent to a shift in the position ofthe EPR line width by 2000 mGauss. In the spectrometer 40 of FIGS. 3 and4, the AFC circuit has been constructed so that the resonator is tunedto the microwave source, using a varactor diode with a range ofapproximately ±8 MHz. Consequently, the microwave frequency is stableand independent of movement of the experimental subject, tissue, orbeing under investigation.

In operation, and with reference to FIG. 3, the magnet 46 applies amagnetic field to the subject under investigation, which is adjacent tothe resonator 42. This magnetic field aligns and separates spins ofunpaired electrons of the subject within the field so that microwaveenergy is absorbed by the subject's molecules. The microwave bridgeoscillator 44 and resonator 42 jointly apply a microwave electromagneticfield to the subject while maintaining a single resonant microwavefrequency in the high Q resonator circuitry, illustrated in FIG. 4. Themicrowave energy is absorbed by the molecules according to a functionaldependence with the magnetic field strength. At one magnetic fieldstrength, the photon energy of the microwave field is matched to theexcited molecular state of the electron spins, and peak absorption isattained. Other frequencies of the EPR resonance are attained bygradually changing, or "sweeping", the strength of the magnetic fieldgenerated by the magnet 46. At the other frequencies, the microwaveabsorption is less. A full sweep by the magnet 46 generates anabsorption spectra having a Lorentzian line-shape, or, more typically,spectra presented as the first derivative of that line shape.

The presence of oxygen in a subject or tissue having a physiologicallyacceptable paramagnetic material, e.g., India ink, affects therelaxation rate of the excited paramagnetic molecule, thus causing anincreased time-integrated intensity, or line-broadening effect withinthe spectra, as discussed above.

FIG. 4 illustrates the external loop resonator 42 constructed inaccordance with the invention and which improves oscillator stabilityand sensitivity for possible resonant mismatching caused by movements ofthe biological tissue. The resonator 42 includes an input 60 forAutomatic Frequency Control (AFC) circuitry, a high frequency input 62for a 50 Ω coaxial line, and a variable inductive coupling 64. Theresonator 42 further has a high Q LC resonant circuit 66, a varactordiode 68, a two-wire λ/2 symmetrical line 70, and a planar loop 72.

The resonator 42 avoids the physical access problems faced byconventional EPR spectrometers in co-locating the resonator and subjectwithin a common magnetic field. The resonator 42 matches and maintainsthe resonant frequency of the resonator cavity by use of a high Q LCcircuit 66 coupled with an external planar loop 72 via a λ/2 symmetricalantenna-like line. The LC circuit 66 is matched to a 50 Ω coaxial lineat the input 62 via a variable inductive coupling 64. The coupling 64consists of a coupling loop, a λ/4 flexible impedance transformer, and amechanism that changes the position of the loop relative to the LCcircuit 66. The application of the impedance transformer makes itpossible to effectively match the resonator to the 50 Ω line. The loopportion 72 is the antennae-like element which is placed in proximity tothe region to be studied. The loop 72 can be configured to optimally fitthe subject, e.g., by going around a protruding tumor, because theresonator need not be in the magnetic field. This is not, however, how aconventional resonator operates, where the subject and the resonator arewithin a common magnetic field, thereby constraining measurementflexibility.

Movement of the subject also influences the resonator's match to the 50Ω coaxial line, which increases the high frequency voltage level at theoutput. This could potentially produce an overload of the preamplifierand detector, and, therefore, the spectrometer 40 of FIG. 3 preferablyutilizes a wide-dynamic preamplifier and detector to measure the EPRabsorption spectra.

The spectrometer 40 described in FIGS. 3 and 4 also operates at a lowerfrequency than conventional EPR spectrometers. Typically, conventionalsystems have oscillating frequencies of approximately 9 GHz, which arestrongly absorbed by high dielectric materials such as water or tissue.Microwave absorption at 9 GHz operates much like a microwave oven,creating unwanted heating in clinical applications. Thus, thespectrometer 40 of FIG. 3 operates with a lower frequency microwaveoscillator. One acceptable frequency range used is within L-bandfrequencies, i.e., 1100-1200 MHz, which provide an acceptable compromisebetween depth penetration and sensitivity. L-band microwave frequenciesare suitable for paramagnetic probes, such as India ink, located atdepths of up to ten millimeters.

However, as those skilled in the art can appreciate, the spectrometer 40is easily constructed according to the invention at lower frequencies,such as within the radiofrequency range of 100 to 1000 MHz, to increasepenetration depth while decreasing sensitivity, which may be desirablein some applications.

Those skilled in the art also understand the principal operation of theother components of the spectrometer 40, FIG. 3, and of other essentialcomponents not illustrated, as they are functionally similar tocomparable, conventional EPR spectrometer components.

The advantages provided by the spectrometer 40 in the context of EPRoximetry using paramagnetic probes are several. First, the spectrometer40 attains maximum possible depth within the target tissue whileretaining sufficient sensitivity for accurate and rapid clinical andbiological applications. The spectrometer 40 further is unaffected bythe particular dimensions of the target tissue, or body, to be studiedbecause the resonator 42 is not limited by the configuration of theresonant structure employed as the detector. Finally, the inevitablemotions of living animals, e.g., heart beats, respiration, and smallphysical movements, are compensated by adjustments to the resonatorfrequency to maintain a balanced bridge.

Thus, the spectrometer 40 of FIG. 3 is especially well-suited for EPRmeasurements of animals or patients when combined with the properties ofphysiologically acceptable paramagnetic materials, such as India ink.This combination in accordance with the invention is suitable for manyclinical and experimental uses for the direct measure of pO₂ in in vivotissues.

In vivo measurements were first conducted in the gastrocnemius musclesof adult mice. A 10 μl slurry of India ink was injected into thesemuscles, whereafter the animals were measured for EPR spectra by an EPRspectrometer, such as the spectrometer 40 of FIG. 3. The coupled planarloop antennae 72, FIG. 4, was positioned over the area of the legcontaining the India ink. When required, blood flow was restricted by aligature around the upper leg. The animals were conscious throughout theexperiment.

The stability of the response of India ink EPR spectra to oxygenconcentration in the mice was studied by measuring the EPR spectrabefore and after restricting the blood flow. FIG. 5 shows the EPR signalspectra 80 of India ink-injected gastrocnemius muscle of the mouse withunrestricted blood flow one day after implantation. When blood flow tothe leg was restricted by a ligation around the upper leg, the EPRspectra response to a reduction of pO₂ is indicated by the narrowingline width and increased line height, as shown by the signal spectra 82.The corresponding pO₂ before and after the constriction of the bloodflow were 11.4 mm Hg and 0.7 mm Hg, respectively.

The kinetics of de-oxygenation in in vivo mouse muscle, subsequent tothe tightening of the tourniquet, was also monitored. FIG. 6 graphicallyshows that the response of India ink is sufficiently rapid to follow thede-oxygenation, typically within 20 seconds. This response lasted for atleast thirty-nine days, as shown by the periodic experimental data ofFIG. 7, with little resultant toxicity, as shown in FIG. 8. The upperdata points of FIGS. 6 and 7 represent unrestricted oxygen flow to themuscle; while the lower data points represent restricted oxygen flow.The multiple, co-located data points represent the several mice tested.

FIGS. 6-8 illustrate the very favorable biological properties of Indiaink, including stability, FIG. 7, low toxicity, FIG. 8, and the rapidresponse of the spectra to changes in pO₂, FIG. 6. Once India ink isinjected into the tissue of interest, pO₂ is measured conveniently,rapidly, and repetitively in a non-invasive manner, i.e., through EPRoximetry. The enormous sensitivity of carbon-based materials, such asIndia ink, to oxygen, combined with its inert physical and chemicalproperties, make carbon-based physiological paramagnetic materials idealprobes for oxygen measurements in tissues, including that of animals andhumans. The paramagnetic material preferably has a substantially uniformparticle size having diameters ranging between about 0.1 μm and about100 μm. The material also preferably includes one relatively largeparticle having a diameter ranging between about 100 μm and about 1.0cm.

India ink, being clinically approved material, can immediately be usedwithin humans to measure oxygen concentrations in clinical settings. TheEPR spectrometer constructed according to the invention, e.g., thespectrometer 40 of FIG. 3, with the external loop resonator andmicrowave bridge, provides clinically effective EPR spectra measurementcapability from paramagnetic materials in living experimental animalsand human subjects. The whole process of measurement in accordance withthe invention takes less than 30 seconds.

The invention offers the additional advantage of providing spatiallyresolved information of pO₂ directly, because the measured EPR spectrais detected at the specific point where the India ink is inserted. Thistechnology is expandable, in accordance with the invention, for thesimultaneous measurement of pO₂ at two or more test sites. A singleparticle of India ink can also be inserted at a selectable spatiallocation within the biological system or tissue to provide a selectableand spatial test probe within the system. The particle is selectedaccording to the test biological system and can be cellular in size,e.g., 0.1 μm, or relatively large in size, e.g., one centimeter. Byinserting such a particle to the system, the EPR spectra is measuredfrom a selectable and localized region in the biological system, such aswithin a cell or within the liver.

EPR oximetry in vivo measurements of a human subject injected withphysiologically acceptable paramagnetic materials were performed throughuse of an extensive tattoo, illustrated in FIG. 9, comprising India ink.The human subject was a volunteer who had the tattoo on his forearm.Accordingly, the EPR spectra of the tattoo indicated the oxygenation ofthe skin. Similar to the experiments conducted on the mice, EPR spectrameasurements were made of the tattooed skin before and afterconstricting the blood flow to the forearm. FIG. 10 graphically showsthe India ink EPR spectra line width variation due to the constrictionof the blood flow, providing a direct measurement of pO₂.

The particular details of the measurements in FIG. 10 are as follows.The forearm with the tattoo was placed between the poles of a magnet ofan L-band microwave spectrometer constructed in accordance with theinvention, such as described in FIG. 3. A prominently black area of thetattoo was positioned on the detector and spectra were obtained beforeand during constriction of the blood flow by means of a rubbertourniquet around the arm and above the tattoo. When the blood flow wasrestricted, the EPR spectra line width narrowed while its line heightincreased. The line width changed from 4050 mGauss, unrestricted, to3400 mGauss, restricted.

Methods and apparatus for determining oxygen concentration in tissuehaving one or more of the foregoing features according to the inventionhave several advantages. These include the ability to directly determineoxygen concentration in in vivo tissues in order to assess their stateand response to therapy. This capability is especially desirable forplanning, and for evaluating tumor therapy and vascular insufficiency.Furthermore, the sensitive, accurate, and repeated measurements of pO₂in tissues provided for by the invention has clinical significance,especially for the optimization and utilization of cancer therapy, andfor the diagnosis and treatment of vascular disease. A number of otherpotential clinical applications, including the evaluation of otherdiseases which concern oxygen pressure within tissues can also benefitfrom the invention by providing clinically useful information. Themodern hospital may eventually utilize the teachings of the invention inan integral clinical role, especially in the oncology and cardiovascularsections of the hospital.

The invention further provides for a wide range of experimental studiesthat may be undertaken in small and large animals. These studies includethe clinical areas described above, and may further include a wide rangeof studies in basic biology and physiology, because of the importance ofoxygen concentrations in most physiological and pathophysiologicalprocesses. The results presented herein, particularly from the EPRstudies of India ink in mice and humans, additionally indicate thatmethods and apparatus in accordance with the invention achieve goodsignal-to-noise ratios and repeatable in vivo EPR measurements, oftenwithout anesthesia. The availability and safety of the paramagneticIndia ink material provide for the immediate and in vivo usage of thesemethods in animals and humans.

India ink has been extensively used in patients as a marker for surgicalprocedures and radiation therapy, in addition to its extensivenon-medical use for decoration. In general surgery, India ink has beenused to mark surgical resection margins. For example tattooing withIndia ink has been described as a precise and practical method foridentifying a biopsy site when there is significant delay between biopsyand definitive surgery. E. Epstein, J. Dermatol, Surg. Oncol. 15, 272(1989). India ink has also been used to indicate the location of lymphnodes and lymphatic channels. For example, Maruyama et al., Nippon GekaGakkai Zasshi 901, 318 (1989), injected India ink in the perigastriclymph nodes of 3,785 patients who had stomach cancer at the operation inorder to find metastatic lymph nodes and reported that this techniquemade it easier to find lymph nodes, thereby improving prognoses. Inradiation therapy, India ink is routinely used to mark fields forirradiation. For example, S. J. Walker, Radiography Today 54, 617(1988), made a survey of methods for marking fields in twelveradiotherapy centers in Britain, and reported that tattooing with Indiaink was a standard procedure in most departments. There was nosuggestion of any serious problems in tattooing. In the endoscopicfield, India ink is used as a long-term colonic mucosal marker. Fennertyet al., The American Journal of Gastroenterology 87, 79 (1992),implanted India ink tattoos to colorectal polygas of patients who werefollowed for at least six months, and reported no side effects orcomplications.

The basis for the apparent lack of toxicity of India Ink is fairlystraight-forward. India ink consists of a suspending vehicle, anemulsifier, and the "active ingredient", which is carbon black. Fromanalyses of its physical properties, and from experience in animals andpatients, the carbon black appears to be both non-reactive andnon-allergenic. The particles of India ink are also very small,homogenous, and independent from each other. When the ink is injectedintravenously, the particles are trapped by the reticuloendothelialsystem, i.e., the liver and spleen, and not in the capillaries of thelung. In vitro experiments have shown that India ink is easily takeninto cells via phagocytosis, without showing any toxicity, as measuredby the colony-forming ability and exclusion of trypan blue. Therefore,in accordance with the invention, India ink is also useful for theselective measurement of intracellular pO₂.

The invention thus attains the objects set forth above, among thoseapparent from preceding description. Since certain changes may be madein the above apparatus and methods without departing from the scope ofthe invention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawing be interpreted asillustrative and not in a limiting sense.

It is also to be understood that the following claims are to cover allgeneric and specific features of the invention described herein, and allstatements of the scope of the invention which, as a matter of language,might be said to fall there between.

Having described the invention, what is claimed and desired to besecured by Letters Patent is:
 1. Apparatus for measuring oxygen tensionin a biological system having a paramagnetic located therein,comprising(a) magnetic means for selectively applying a magnetic fieldof selectable strength to the biological system, (b) electromagneticmeans for selectively applying electromagnetic radiation having aselected substantially constant frequency to the paramagnetic materialin the biological system, (c) detection means for detecting the electronparamagnetic spectra of the biological system, said spectra having aselected peak-to-peak line width that is indicative of said oxygentension in the biological system, (d) resonator means coupled to theelectromagnetic means,said resonator means including means for tuningsaid resonator means to the constant frequency of said electromagneticmeans to maintain a substantially constant resonant frequency inresponse to movements in the biological system, (e) console means incommunication with said detection means for displaying said spectra, and(f) computer means connected to said console means for controlling saidapparatus, and for analyzing said spectra to determine said oxygentension.
 2. Apparatus according to claim 1 wherein said detection meansincludes preamplifier means and a detector for combined, high-dynamicrange detection of said electron paramagnetic spectra.
 3. Apparatusaccording to claim 1 wherein said tuning means comprises an automaticfrequency control means with a fixed frequency oscillator and a varactordiode tuned resonator.
 4. Apparatus according to claim 1 wherein saidresonator means comprises a high Q LC circuit coupled with an externalplanar loop via a λ/2 symmetrical line.
 5. A system for determiningoxygen tension within a biological system, comprisingmeans forintroducing india ink into the biological system, magnetic means forapplying a first magnetic field to the biological system, radiationmeans for applying electromagnetic radiation having a frequency betweenabout 100 MHz and about 5 GHz to the biological system to excite theindia ink to a higher energy state, said excited india ink then relaxingat a rate dependent upon the presence of oxygen in the biologicalsystem, and means for determining the electron paramagnetic resonancespectra of the biological system, said spectra being indicative of theoxygen tension within the biological system.
 6. The system of claim 5further comprising means for sweeping the magnitude of said firstmagnetic field between about 100 Gauss and about 500 Gauss.
 7. Thesystem of claim 6 wherein said means for sweeping includessecondmagnetic means for applying a second magnetic field generally parallelto the first magnetic field to the biological system, and adjustingmeans for adjusting the intensity of the second magnetic field to varythe magnitude of the first magnetic field.
 8. The system of claim 6wherein said means for sweeping comprises a magnet or an electromagnet.9. The system of claim 5 further comprising modulation means forgenerating a third magnetic field generally orthogonal to said firstmagnetic field, and means for modulating said third magnetic fieldbetween about 1 KHz and about 500 KHz.
 10. The system of claim 5 whereinsaid means for determining comprises a resonator having an operatingresonant frequency.
 11. The system of claim 10 wherein said radiationmeans applies radiation having a substantially constant frequency, saidsystem further comprising resonant frequency control means for adjustingsaid resonant frequency of said resonator in response to movements ofthe biological system to match said constant frequency of said radiationmeans.
 12. The system of claim 5 further including a resonator operableat a selected resonant frequency.
 13. The system of claim 12 whereinsaid radiation means applies radiation having a substantially constantfrequency, said system further including tuning means for tuning saidresonant frequency of said resonator to said constant frequency of saidradiation means to maintain a substantially constant resonant frequency.14. The system of claim 13 wherein said tuning means includes anautomatic frequency control circuit.
 15. The system of claim 12 whereinsaid resonator includes an LC resonant circuit, and an external planarantenna loop in communication with said resonant circuit.
 16. The systemof claim 15 wherein said resonator further includes a variable inductivecoupling and a varactor diode in electrical communication with saidresonant circuit.
 17. A system for determining oxygen tension within abiological system, comprisingmeans for introducing an electronparamagnetic material into the biological system, magnetic means forapplying a magnetic field to the biological system, radiation means forapplying electromagnetic radiation having a selected substantiallyconstant frequency to the biological system to excite the paramagneticmaterial, resonator means operable at a selected resonant frequency,tuning means for tuning said resonant frequency of said resonator tomatch said constant frequency radiation means to maintain a constantresonant frequency in response to movements in the biological system,and means for determining the electron paramagnetic resonance spectra ofthe biological system, said spectra being indicative of the oxygentension within the biological system.
 18. The system of claim 17 whereinsaid electron paramagnetic material comprises carbon black.